[4] Südostbahn (SOB), a private company primarily owned by the federal government and the canton of St. Gallen, operates the service.

Between 2013–2023 it operated over a circular route (German: Ringzug), also running over the Ziegelbrücke–Sargans railway line south of the Walensee.

[7] Südostbahn introduced the new S4 on 15 December 2013 as a circle line (German: Ringzug), which was sometimes referred to as the 'Alpstein round trip' (Alpsteinrundfahrt).

In Uznach, which lies to the west of the junction between the Uznach–Wattwil and Rapperswil–Ziegelbrücke lines, the trains reversed their direction of travel before continuing on the circle route.

[10] Rerouting of the S4 and introduction of the new S17 service were made possible through the expansion to dual track between Uznach and Schmerikon (constructed between 2021 and 2023) on the Rapperswil–Ziegelbrücke railway line.